The present invention relates to acicular, cobalt-modified iron oxides, consisting of a core of an iron oxide of the formula FeO.sub.x where x is from 1.33 to 1.5 and a ferrite shell which contains a total amount of from 0.4 to 12% by weight of cobalt ions and not more than 5% by weight of iron(II) ions, the percentages being based on the total amount of the material, and which have improved aging resistance, particularly with regard to the coercive force, and a process for their preparation.
Cobalt-modified, acicular gamma-iron(III) oxide has been used for a long time and in large amounts as magnetizable material in the production of magnetic recording media. The thermal and mechanical instability of residual induction and coercive force originally associated with cobalt doping of iron(III) oxide was substantially reduced when, instead of uniform volume doping, doping was effected in such a way that the cobalt is present in the form of a magnetic cobalt compound in an outer shell on an essentially cobalt-free core of iron(III) oxide or berthollide iron oxides. Several processes have been described for the preparation of such pigments having a layer structure. These processes essentially involve the application of a cobalt-containing compound to magnetic acicular iron oxide and subsequent heating of the coated oxides, as described in, for example, GB-B 1 441 185 or U.S. Pat. No. 3 573 980. However, if it is intended to avoid the disadvantages of volume doping described above, only limited increases in the coercive force can be achieved using these processes. Application of a mixture of Fe(II) hydroxide and Co hydroxide with simultaneous oxidation of the coating is also known (DE-A 22 35 383). Although the coercive force can be increased to a greater extent by this method, relatively large amounts of cobalt are required for this purpose and the degree of utilization of the cobalt is therefore low. Furthermore, EP-A 14 902 and EP-A 14 903 disclose that a mixture of Fe(II) hydroxide and Co hydroxide can be applied to the gamma-iron(III) oxide with simultaneous introduction of inert gas in order to prevent oxidation of the divalent iron. The coercive force can be increased to a greater extent with a lower cobalt content by this process. However, particularly pronounced core/shell structures of the particles prepared in this way are not achieved, so that the disadvantages described in connection with volume doping once again occur, particularly in the case of pigments having a relatively high coercive force. This is due to the fact that the high Fe(II) concentrations required for high coercive forces in this process greatly accelerate diffusion of the cobalt into the interior of the particle.
Pigments having a particularly pronounced core/shell structure can be obtained if a process described in U.S. Pat. No. 4,770,903 is used. In this process, the gamma-iron oxide used for coating is first coated with Fe(OH).sub.2 under inert gas in order to form a magnetite-like surface layer, on which cobalt ferrite is crystallized in the second step under oxidizing conditions at low temperatures.
Regardless of the particular, more or less advantageous preparation process, however, all cobalt-doped iron oxides tend to undergo changes, usually a decrease in the coercive force, if the pigment powders are stored for a long time. To improve this property, JP-A 60 196 905 proposed coating cobalt-modified iron oxides with alkyl- or alkenylsilanes. However, the stated compounds are insoluble in water, so that the pigment to be coated has to be first dried and then redispersed in organic solvents. On the other hand, DE-A 32 28 021 proposes hydrothermal aftertreatment for improving the long-term stability of cobalt-doped iron oxides. However, this entails a very expensive process. Finally, DE-A 33 12 243 states that coating with Mn, Zn or V compounds has a stability-improving effect, but does not specify this in any more detail.
It is an object of the present invention to provide a cobalt-doped iron oxide which has long-term stability, and a simple process for its preparation.